The objective of this project is to examine the physiological and pharmacological mechanisms involved in the centrifugal regulation of primary afferent input. This work is being pursued, in the current program, at two levels: 1) The regulation of afferent terminal polarization by neuronal systems in the dorsal horn with respect to various putative transmitters present in this structure. At present the focus is on the site of action of serotonin and its interaction with other putative neurotransmitters in the dorsal horn (e.g., glutamate, GABA and norepinephrine). 2) A second focus will be on identifying the sensory modality associated with nerves exhibiting DRP. 3) The other major objective will be to determine the effect of antidromic impulses on receptor sensitivity to natural stimuli.